Climate Change, Coming Home: Global warming’s effects on populations

Climate Change, Coming Home

Global warming's effects on
populations

Since the 1970s, rainfall has been
scarce in the Sahel, the wide belt of semi-arid land that stretches across
Africa on the southern edge of the Sahara Desert. One of the worst-affected
areas has been the Tigray region of northern Ethiopia, where a series of
prolonged droughts exacerbated by war caused widespread famine in the 1970s and
1980s.

To help increase the productivity
of farmers' fields, the local government decided in the late 1980s to build a
series of small dams to trap the unreliable rainfall and connect these to
simple irrigation systems. Sure enough, harvests increased and fewer people
went hungry-but health researchers also found that children in villages near
the dams were seven times as likely to suffer from malaria. The water stored
behind the dams provided perfect breeding habitat for the mosquitoes that carry
the disease.

The people of this isolated rural
region of Ethiopia offer a glimpse into the human future-a view of how global
climate change can play havoc with populations' lives and livelihoods, and how
addressing one climate-related problem can sometimes cause another. The World
Health Organization (WHO) has calculated that by 2020 human-triggered climate
change could kill 300,000 people worldwide every year. By 2000, in fact,
climate change was already responsible for 150,000 excess deaths
annually-deaths that wouldn't have occurred if we humans weren't burning vast
quantities of fossil fuels and loading up the air with carbon dioxide and other
greenhouse gases.

Jonathan Patz, a professor with the
University of Wisconsin/Madison's Center for Sustainability and the Global
Environment, praises the WHO's sober accounting as the most comprehensive, scientific
estimate available of the health effects of climate change. The agency combined
models of recent and projected climate change with data on several health
dangers that are known to be affected by climate (including malaria, diarrheal
diseases, and malnutrition) to calculate the disease burden due to changes in
climate. However, Patz says, "their estimate is extremely
conservative." Not only are the underlying assumptions conservative, but the
analysis only concerns a few of the relatively better-understood health risks
of climate change.

Climate change might have a few
pluses for our species-for example, warmer winters probably mean fewer
cold-related deaths in North America and Europe, while in some parts of the
tropics hotter and drier conditions could reduce the survival of
disease-carrying mosquitoes. But most of the effects of climate change are
likely to be harmful ones: declining agricultural production and more hungry
people, increased spread of infectious diseases, dangerous heat waves and floods.
Although no region of the globe will be entirely spared, the negative effects
are likely to fall most heavily on poor nations in tropical and subtropical
regions. In other words, the people most vulnerable to the effects of climate
change are precisely those who are least responsible for causing it-and those
who have the least resources with which to adapt to it.

Food

"Malnutrition will very likely be
one of the biggest impacts in low-income countries," says Kristie Ebi, an
environmental consultant who has served on the Intergovernmental Panel on
Climate Change (IPCC) and several other climate-change-related scientific
bodies. Globally, food production is likely to decrease only modestly at worst,
but this overall pattern hides what many researchers see as a growing
inequality between the haves and have-nots of the world.

Some relatively wealthy countries
in temperate regions will likely see crop yields rise, mainly due to longer,
warmer growing seasons. Even the excess carbon dioxide in the air that is the
underlying cause of climate change can theoretically be a boon for agriculture,
acting as a fertilizer when other conditions for plant growth are favorable.
Though it's not yet clear whether or how this effect of carbon dioxide will
play out in the real world, any beneficial effects are most likely to be seen
at middle and high latitudes. The prospect of these changes causes skeptics of
climate-change doom and gloom to envision vast stretches of northern tundra
transformed into a future breadbasket.

Meanwhile, however, crop yields are
likely to fall in the tropical and subtropical world, latitudes with many
poorer countries where most of the world's hungry and malnourished live today.
There, many crops are already growing near the upper bound of their temperature
tolerance, so further warming would push them beyond their limits. In some
areas, precipitation may increase, causing crops to rot; elsewhere, rainfall
may diminish and become more erratic, arriving unpredictably as intense
downpours that will run off the parched earth instead of nourishing the soil.
And since the economies of many poorer countries are heavily dependent on
agriculture, the failure of crops at home will leave them unable to buy surplus
grain abroad.

For the last decade and a half,
Martin Parry of the U.K. Met (formerly Meteorological) Office (and current
co-chair of the IPCC working group on impacts of climate change), Cynthia
Rosenzweig of the Goddard Institute for Space Studies, and a large group of
other researchers from various institutions have been modeling the possible
effects of climate change on production of the world's staple grain crops:
wheat, rice, maize, and soybeans. Their work integrates several complex
computer models-of global climate, crop yields, world food trade, and various
patterns of economic development and population growth-to predict future global
agricultural production and the risk of hunger. One set of their calculations
indicates that, accounting for future population growth, continued "business as
usual" greenhouse gas emissions would increase the ranks of the hungry by 80
million by 2080, mostly in Africa and southern Asia.

Already there are hints that such
projections are beginning to come true. Recently, Ebi worked on a U.S. Agency
for International Development study of possible adaptations to climate change
in Zignasso, a town of about 3,000 people in the main agricultural area of
southern Mali. "It's gotten hotter. It's gotten drier," she says of the area's
climate. "Farmers are seeing the rains come at somewhat different times of
year," and sometimes there are dry spells during the rainy season. To combat
declining soil quality farmers have started adding more fertilizer to their
fields of potatoes, the main cash crop, but nevertheless climate shifts mean
that harvests are getting smaller.

If average temperatures in this
area of Mali increase, as expected, another 2-3°C by the 2060s, potato yields
could further decrease by about a quarter. It's not clear whether rainfall in
the area will increase or decrease. In general, scientists simply don't
understand this part of the climate system very well, so different models often
disagree in their predictions. But in either case, says Ebi, the soil will
probably be drier, because warm temperatures cause soil moisture to evaporate
more quickly-and that could spell trouble for rice, the area's staple grain
crop.

As drier conditions shrink growing
seasons in tropical and subtropical environments, farmers have been encouraged
by governments and aid agencies to turn to new, fast-growing crop varieties:
rice that matures in 90 days, for example, rather than 120 days. But this
solution may beget its own set of problems, Ebi says: "The fast-growing
cultivars, because they grow faster, have less time to absorb micronutrients"
from the soil, so they might provide people with sufficient calories but leave
them vulnerable to vitamin and mineral deficiencies. "We need to pay attention
to the quality of the food, not just the quantity."

Disease

Much research on climate change and
infectious disease has focused on vector-borne diseases, in which a pathogen is
carried from one human host to another by a third species, often a mosquito or
other type of insect. Common vector-borne diseases in developing countries
include malaria and dengue fever, both transmitted by mosquitoes, and in
developed countries Lyme disease, transmitted by ticks. Many vector-borne
diseases are sensitive to shortterm climate variations and show seasonal
patterns of transmission and year-to-year differences depending on the weather.
Warmer weather (at least up to a certain maximum) increases the rate of
mosquito reproduction, increases the number of blood meals a mosquito takes,
prolongs the mosquito breeding season, and makes disease-causing microorganisms
reproduce or mature more quickly.

However, linking observed increases
in vector-borne diseases to changing weather patterns has so far proved
difficult. "If we look over time, to see whether the changing distribution of
malaria, for example, is due to climate change, it's difficult if not
impossible to demonstrate that because many other things have changed over the
same period," says Andrew Haines, director of the London School of Hygiene and
Tropical Medicine. In the highlands of East Africa, for example, the onset of
warmer weather has also been accompanied by other factors that can increase
malaria, including drug resistance of the malaria parasite, human migration and
changes in immunity, and the failure of mosquito-control programs. "The other problem
is that we often don't have very good data over long time periods, particularly
in those parts of the world where one might expect an impact from climate
change," Haines adds.

Still, scientists have observed
many warm-adapted species of plants and animals moving to higher altitudes and
latitudes as temperatures have climbed in recent years. "To think that bugs and
mosquitoes are immune to [this migration] is wishful thinking," says Paul
Epstein, associate director of the Center for Health and the Global Environment
at Harvard Medical School. In fact, the mosquitoes that carry malaria and
dengue fever have recently been spotted in highland areas of Africa, Asia, and
Latin America where temperatures are increasing, glaciers are retreating, and
plant communities are moving upward, and the Lyme disease tick has migrated to
higher latitudes in Sweden and higher altitudes in the Czech Republic as the
climate has warmed. About 45 percent of the world's population currently lives
in areas where the climate is potentially suitable for malaria transmission,
and as the planet continues to warm that proportion could increase to 60
percent by century's end.

There is a difference, though,
between the expanding range of a vector species
and the expanding range of a vector-borne disease.
Duane Gubler, former director of the U.S. Centers for Disease Control Division
of Vector-Borne Infectious Diseases, points out that malaria was once endemic
in large portions of the United States. In fact the mosquitoes that carry the
disease are still present and the climate today is already suitable for
transmission of the disease during the summer months. "Unless our public health
system and our standard of living tank, we're not going to see those huge
outbreaks," he says. He also cautions that temperature is not the only factor
that influences the survival of disease-carrying mosquitoes, and elsewhere in
the world many other factors in addition to climate contribute to recent
disease outbreaks.

Indeed, many scientists agree with
Gubler that good public health systems and the ready availability of medical
care, assuming they are not neglected, are likely to protect the United States
and many other wealthy nations from widespread outbreaks of malaria and other
vector-borne tropical diseases. By contrast, malaria may be more likely to take
hold in temperate regions where the public health infrastructure has
deteriorated in recent years, such as southern portions of the former Soviet
Union.

The populations most at risk from
the spread of malaria may be those at the margins of the disease's present
distribution in developing countries without good access to health care. As
malaria invades these new areas, its effects may become more severe. "When you
have an outbreak in an area where people are not immune, they've not been
exposed to malaria regularly, mortality can be 20 or 30 percent," Kristie Ebi
says-compared to about 3 percent in areas where the disease is long
established. Moreover, because malaria is such a common disease-infecting half
a billion people each year and killing 1 to 2 million-a very slight increase in
the relative risk of the disease can translate into hundreds of thousands of
additional cases.

Many other climate/disease links
have also been noted. Higher-than-average temperatures often lead to
higher-than-average rates of food poisoning. Where access to clean water is
scarce, droughts often trigger outbreaks of diseases associated with poor
hygiene, such as diarrheal diseases, scabies, conjunctivitis, and trachoma. Floods,
too, can increase the risk of diarrheal diseases by contaminating waterways and
drinking-water supplies with human and animal wastes. Storms often leave
infectious disease in their wake; in 2000, a series of heavy rains and three
cyclones over the course of six weeks quintupled malaria infections in
Mozambique.

Additional research on climate and
infectious disease has focused on El Niño events, periods of higher sea-surface
temperature in the southern Pacific that affect weather throughout the world.
Health consequences linked to El Niño events include cholera outbreaks in
Bangladesh; malaria in South America, the Punjab region of India, and elsewhere
in Asia; rift valley fever in East Africa; Ross River virus in Australia; and
hantavirus pulmonary syndrome in the southwestern United States. During the
1997-8 El Niño event, winter temperatures were about 5°C warmer than normal in
Lima, Peru, and hospital admissions for diarrhea increased by 200 percent. Some
climate-change models predict that El Niño events will become more frequent and
more intense in the future, so this phenomenon is not only a model of how
short-term climate variation affects infectious disease, but also a potential
effect of climate change itself.

Weather extremes and sea levels

"Climate change has two parts-it's
the warming and then the extremes," Paul Epstein says. The planet's weather is
expected to become not only warmer on average, but more variable, with more
frequent and intense heat waves, droughts, and torrential rains. Warmer air
holds more moisture, so the global hydrologic cycle is expected to accelerate
and intensify, leading to violent storms and stronger hurricanes. In addition
to their effects on infectious diseases, such extremes of weather pose direct
physical risks to the humans in their path-heat stroke, drowning, dehydration,
injury.

In general, while patterns of
weather can be linked to climate change, it's difficult to ascribe any one
extreme event to that cause. For example, Hurricane Katrina, the August 2005
storm that slammed into the U.S. Gulf Coast and inundated New Orleans, is the kind of storm we would expect to see
more frequently with climate change, but scientists can't say for certain that
climate change caused that particular
event.

Despite that general difficulty,
Peter Stott and colleagues at the Hadley Centre for Climate Prediction and
Research and from Oxford University have taken a careful statistical approach
to demonstrate the links between climate change and the European summer heat
wave of 2003. That summer is thought to have been the hottest in Europe since
1500, and in August over 20,000 people died in France and thousands more
elsewhere in Europe. Stott and his colleagues demonstrated that the magnitude
and timing of the heat wave were consistent with computer models of climate
change, and calculated that the probability of such a heat wave was doubled by
the amount of global warming to date.

Many deaths during heat waves occur
among the very old, the very young, and frail individuals, especially those
with underlying cardiovascular and respiratory diseases. However, Stott's team
also found that mortality rates in hard-hit areas did not dip below normal
after the 2003 heat wave. In other words, the victims represented excess
deaths, not merely deaths that would likely have occurred soon anyhow.

Rising sea levels also represent a
direct physical threat from climate change. Some coastal populations will be
threatened by inundation from the water's slow, inexorable rise, while even
larger areas will be subject to periodic danger from intensified storm surges.
The Intergovernmental Panel on Climate Change, which includes scientists from
113 countries, is charged with reviewing and synthesizing research on climate
change and its effects. The IPCC predicts that global sea levels will rise
18-59 centimeters over the next century. Most of that calculation represents
the expansion of water as sea surface temperatures warm; melting of polar ice
will also contribute, but the panel says the details of this process are not
understood well enough to quantify.

Small, low-lying Pacific island
nations are likely to be the first affected by rising seas. Already the
citizens of Tuvalu, a group of small reef islands and atolls in the Pacific,
are making plans to evacuate en masse over the next decade. A sea-level rise of
one meter-an extreme case, but not outside the realm of possibility over the
long term-could drive 18.6 million people in China, 13 million in Bangladesh,
3.5 million in Egypt, and 3.3 million in Indonesia from their homes.

The best estimates so far suggest
that climate change will cause many excess deaths but not slow population
growth. (A study due from the IPCC in April reportedly projects "millions"
starving from food shortages by 2080.) The planet's population will continue to
rise, to perhaps 9 billion sometime this century, according to a
middle-of-the-road United Nations projection.

Climate change and population are
interlinked in complex ways. Most obviously, population growth worsens climate
change-more people on the planet means more carbon dioxide emissions. And Parry
and Rosenzweig's modeling of food security indicates that reducing the rate of
global population growth would do more to reduce the number of hungry people in
the world than would limiting climate change.

At the same time, climate change
makes clear that an overcrowded planet isn't a matter of numbers alone. By
reducing the planet's ability to feed us, stretching already overburdened
public health resources, and making parts of the globe uninhabitable
altogether, climate change will make a given level of population "feel" more
crowded.

These ironies only deepen when you
consider that the level of development that will help many wealthy countries
avoid some of the adverse effects of climate change itself depends on fossil
fuels. "A substantial fraction of what we call development is in fact
adaptation to climate," explains Gerry Stokes, vice president of international
partnerships for Battelle Memorial Institute. The clothes we wear, how we build
our houses, what crops we plant, the public health systems we design-all vary
from place to place, depending on the climate patterns we've observed in the
past. People in the poorest, least developed countries "are incredibly
vulnerable to the current climate, and the principle of minimum astonishment
says they're probably not particularly well adapted to future climate either."

Scientists think about some of
these tradeoffs and interconnections with the help of the Kaya equation (named
for the Japanese scientist Yoichi Kaya):

Currently, the major factor contributing
to increased emissions is not population growth but growth in affluence,
particularly in rapidly modernizing countries such as India and China. During
the 20th century the global population quadrupled, but carbon emissions
increased 12-fold. Of course, it's morally dicey-not to mention politically
impossible-to suggest that countries should reduce their populations or that
poor countries should remain poor. (And although it is beyond the scope of this
article, some have argued that immigration patterns can be critical as well,
when people move from poor countries to rich ones and eventually become
integrated as members of those high-consuming societies; this topic is equally
complex and delicate.) Scientists and policymakers primarily focus on the other
two terms of the equation to reduce carbon dioxide emissions and thus mitigate
climate change-that is, improving the energy-efficiency of economies, and
finding low-carbon sources of energy.

Many technologies already exist to
accomplish both of these goals, although they are often more expensive than
fossil-fuel intensive forms of energy (in part because of longterm government
subsidies to fossil fuel industries and because fossil fuel prices do not
reflect the associated costs of climate change and other environmental ills).
Often, it makes the most economic sense to apply these technologies first in
developing countries-it's better to build a super-efficient power plant in a
new area than to tear down or retrofit an existing one. Stokes says wealthy nations
could atone for some of their carbon emissions by helping developing countries
bear the extra cost of such climate-friendly development.

Focusing such efforts on developing
countries could also produce the greatest gains in resilience, or ability to
adapt to climate change. About 2 billion of the world's people lack access to
electricity-but if this energy could be made available to them in cleanly
generated form, they could, for example, prevent food from spoiling, store
vaccines, and purify water. "By providing cleaner energy for populations...you
can also get near-term benefits to health by reducing air pollution," Haines
adds.

Maybe that explains why many
scientists who have studied the health effects of climate change seem
optimistic in the end. "I believe that this climate crisis is going to lead us
to a clean energy transition, and that this can be the engine of growth for the
21st century and help people develop cleanly and sustainably," Epstein says.

Sarah
DeWeerdt is a Seattle-based science writer
specializing in biology and the environment.